The present invention relates to a battery comprising a cathode, an anode and an electrolyte solution, and an electrode used in the battery.
In recent years, reduction in size and weight of portable electric devices typified by cellular phones, PDAs (personal digital assistants) or laptop computers has been vigorously pursued, and as part of the reduction, an improvement in energy density of batteries, specifically secondary batteries as power sources for the devices has been strongly required.
One example of a secondary battery which can obtain a high energy density is a lithium-ion secondary battery using a material capable of inserting and extracting lithium (Li) such as a carbon material for an anode. The lithium-ion secondary battery is designed so that lithium inserted into an anode material is always in an ion state, so the energy density is highly dependent on the number of lithium ions capable of being inserted into the anode material. Therefore, in the lithium-ion secondary battery, it is expected that when the amount of insertion of lithium is increased, the energy density can be further improved. However, the amount of insertion of graphite, which is considered at present to be a material capable of the most effectively inserting and extracting lithium ions is theoretically limited to 372 mAh per gram on an electricity amount basis, and recently the amount of insertion of graphite has been approaching the limit by active development.
Another example of the secondary battery capable of obtaining a high energy density is a lithium secondary battery using lithium metal for an anode, and using only precipitation and dissolution reactions of lithium metal for an anode reaction. In the lithium secondary battery, a theoretical electrochemical equivalent of the lithium metal is as large as 2054 mAh/cm3, which is 2.5 times larger than that of graphite used in the lithium-ion secondary battery, so it is expected that the lithium secondary battery can obtain a much higher energy density than the lithium-ion secondary battery. A large number of researchers have been conducting research and development aimed at putting the lithium secondary battery to practical use (for example, Lithium Batteries edited by Jean-Paul Gabano, Academic Press, 1983, London, N.Y.).
However, the lithium secondary battery has a problem that when a charge-discharge cycle is repeated, a large decline in its discharge capacity occurs, so it is difficult to put the lithium secondary battery to practical use. The decline in the capacity occurs because the lithium secondary battery uses precipitation-dissolution reactions of the lithium metal in the anode. In accordance with charge and discharge, the volume of the anode largely increases or decreases by the amount of the capacity corresponding to lithium ions transferred between the cathode and the anode, so the volume of the anode is largely changed, thereby it is difficult for a dissolution reaction and a recrystallization reaction of a lithium metal crystal to reversibly proceed. Further, the higher energy density the lithium secondary battery achieves, the more largely the volume of the anode is changed, and the more pronouncedly the capacity declines. Moreover, falling off of precipitated lithium, or a loss of the precipitated lithium because the lithium forms a coating with an electrolyte solution is considered as a cause of the decline in the capacity.
Therefore, the applicant of the invention have developed a novel secondary battery in which the capacity of the anode includes a capacity component by insertion and extraction of lithium and a capacity component by precipitation and dissolution of lithium, and is represented by the sum of them (refer to International Publication No. WO 01/22519 A1). In the secondary battery, a carbon material capable of inserting and extracting lithium is used for the anode, and lithium is precipitated on a surface of the carbon material during charge. The secondary battery holds promise of improving charge-discharge cycle characteristics while achieving a higher energy density.
However, like the lithium secondary battery, the secondary battery uses precipitation-dissolution reactions of lithium, so the secondary battery has a problem that when a charge-discharge cycle is repeated, a larger decline in the discharge capacity occurs, compared to the lithium-ion secondary battery. In order to overcome the problem, it is considered that it is important to uniformly precipitate lithium on the whole anode. For the purpose, it is required to contrive the structure of the anode.
In a conventional lithium-ion secondary battery, a large number of structural contrivances for improving characteristics have been reported. For example, in Japanese Unexamined Patent Application Publication No. Hei 10-270016, a method of improving liquid absorption speed in a surface of an electrode through forming a continuous shallow groove on the surface of the electrode is reported, and in Japanese Unexamined Patent Application Publication No. Hei 10-97863, a method of improving liquid absorption speed of an electrode through setting porosity of the electrode.
However, in the method disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-270016, the liquid absorption speed in the surface of the electrode can be improved, but it is difficult to improve liquid absorption speed in the whole electrode, thereby it is difficult to obtain sufficient characteristics. Further, in the method disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-97863, the liquid absorption speed of the electrode can be improved, but the volume density of the electrode is sacrificed, so it is difficult to obtain a high energy density.
In view of the foregoing, it is an object of the invention to provide an electrode and a battery both having superior charge-discharge cycle characteristics and capable of obtaining a higher energy density.